Higher bandwidth and lower latency for web and streaming media clients dependent on remote central servers can be accomplished by providing caching servers at more local points in the network that keep copies of files previously retrieved from the remote server for subsequent repeated access by the local clients. Multiple caching servers can be arranged in hierarchical fashion to share a common cache of objects, in effect forming a shared second level cache of objects in response to aggregated requests for files that were not available from a respective server's first level cache. The theory underlying “caching” is that since the same file may be used more than once, it may be more efficient (both in terms of speed and resource utilization) to keep a copy locally rather than retrieve it a second time from a remote source. Typically, each caching server caches a small set of “hot” recently accessed objects in a fast and relatively expensive random access memory attached to its internal bus, and a somewhat larger set of such objects in a slower and cheaper random access peripheral storage device such as a magnetic or optical disk.
An even bigger set of objects can be locally cached in a networked storage medium such as a SAN (“Storage Area Network”) or a NAS (“Network Attached Storage”) that is accessible by the caching server (and also by other local clients and/or servers) via a relatively short and fast transmission medium such as fiber optic cable that offers higher bandwidth and lower latency than is available from the remote server.
Prefetching is a known technique for analyzing current and/or past file requests to predict what files are likely to be requested in the future, and uses those predictions to retrieve the files from a remote server on a less urgent basis before they are actually requested, thereby reducing not only latency (delay between the request and the response) but also network congestion. It differs from caching in that the focus is not on whether to keep a local copy of a file that has already been retrieved or updated (which is mostly a question of how best to use the available local storage capacity) but rather on whether to obtain from the remote server a file that is not currently available locally and that is not currently the subject of any pending requests (which is mostly a question of how best to use the available transmission capacity to the remote server).